As in ordinary vehicles, two-wheeled motor vehicles have a silencer or muffler provided in an exhaust passage. The muffler can offer certain reduction in noise produced by the exhaust on the motor vehicle. However, while traveling in a densely populated urban area, the two-wheeled motor vehicle is required to achieve a further noise reduction than as it achieves during the travel in a sparsely populated suburban area.
In general, the length and flow-passage area of an exhaust passage are determined based on the rated output of an engine. Accordingly, it may occur that when the two-wheeled motor vehicle is traveling at a low speed with low engine output, the length or the flow-passage area of the exhaust passage becomes excessively large and the engine efficiency is reduced. To avoid this problem, a prior technology relying on the use of an exhaust valve has been proposed. The exhaust valve is disposed in an exhaust passage and operable to reduce the flow-passage area or the length of the exhaust passage when the engine power output is small, thereby preventing a reduction in the engine efficiency.
An exhaust valve so configured as to reduce the cross-sectional area of a flow passage promises a certain level of noise reduction effect, as will be discussed below.
Exhaust noise produced by the engine is emitted along an exhaust passage. When the exhaust valve closes the exhaust passage, part of the exhaust noise is blocked from escaping to the outside by the exhaust valve. A certain level of noise reduction effect can thus be attained.
Such exhaust valve is disclosed in, for example, Japanese Patent Publication (JP-B2) No. 3242240. The exhaust valve disclosed in JP 3242240 B2 is disposed in an intermediate part of the exhaust passage of a two-wheeled motor vehicle. The degree of opening of the exhaust valve is proportional to the rotation angle of a throttle grip of the two-wheeled motor vehicle.
The relation between the rotation angle of the throttle grip and the degree of opening of the exhaust valve is that when the rotation angle of the throttle grip increase from zero to a predetermined angle, the degree of opening of the exhaust valve is approximately proportional to the rotation angle of the throttle grip. Due to such proportional relation, the exhaust valve begins to open simultaneously with the start of rotation of the throttle grip. With this arrangement, the noise becomes large even when the two-wheeled motor vehicle is traveling at a low constant speed.
In view of the travel in a closely populated urban area, it is highly desirable that the noise produced from an engine of the two-wheeled motor vehicle during the travel at a low constant speed is as low as possible.